[Stage 0.3] Separate the v1 and v2 CI/release paths

[draedful]

Parent: #310

Goal

Make the v1 and v2 delivery paths independent and prevent a release from publishing the wrong major version.

Scope

• Run the complete CI suite for pull requests and relevant pushes on both main and release/v1.
• Restrict release workflow triggers to the intended release branches.
• Add a release guard that permits only 1.x from release/v1.
• Add a release guard that prevents 1.x from being published from main.
• Configure the npm dist-tags according to the policy defined in Stage 0.1.
• Keep v2 prereleases on a non-stable tag such as next, or disable the main release until the v2 release flow is intentionally enabled.
• Confirm that the release GitHub App can operate under the future branch rulesets.

Acceptance criteria

• Pull requests to both branches run the agreed required checks.
• A 1.x release attempt from main fails before publishing.
• A non-1.x release attempt from release/v1 fails before publishing.
• Only the intended branches can invoke npm publication.
• The selected npm dist-tags are covered by an automated or documented verification.
• The workflow has a safe dry-run or equivalent way to validate routing without an accidental publication.